Both are containers, not codecs
This is the distinction that makes the rest of the comparison make sense. A container is a box: it holds one or more video tracks, audio tracks, subtitles and chapter markers, and records how they line up in time. The codec is the compression applied to the video inside the box — H.264, H.265, VP9, AV1.
So "MP4 vs MKV" is not a quality question. An H.264 stream in an MKV and the same H.264 stream in an MP4 are identical video. Asking which looks better is like asking whether a document is sharper as a .zip or a .tar.
What MKV can do that MP4 can't
MKV is an open, flexible container designed to hold almost anything. That flexibility is exactly why it is the format of choice for archival copies and for anything with multiple languages.
- Carry any codec, including ones MP4 was never specified for.
- Hold several audio tracks and several subtitle tracks cleanly — the usual reason film rips are MKV.
- Store subtitles as selectable text rather than burning them into the picture.
- Keep chapter markers and rich metadata that survive most players.
What MP4 can do that MKV can't
Play essentially everywhere. MP4 with H.264 has hardware decoding in nearly every phone, TV, console, browser and editing application made in the last decade and a half. MKV does not: plenty of smart TVs, older phones, messaging apps and social platforms simply refuse it.
MP4 also streams properly over the web. Its index can be placed at the front of the file, so a browser can begin playback before the download finishes. That is why MP4 is what platforms accept and MKV generally is not.
Which should you use?
- Keeping a master copy, or a film with several audio and subtitle tracks — MKV.
- Sending it to someone, posting it, editing it, or playing it on a TV — MP4.
- Unsure — MP4. The compatibility is worth more than the flexibility to almost everyone.
Converting between them usually needs no re-encoding
Because the video inside is often already H.264 or H.265 — codecs both containers accept — moving from MKV to MP4 can be a remux rather than a conversion. The existing video and audio streams are copied into the new container untouched: no quality loss at all, and it finishes in seconds rather than minutes, because nothing is being compressed again.
Re-encoding is only genuinely necessary when the MKV holds something MP4 will not accept — VP9 video, for instance, or certain subtitle formats. The converter here picks remuxing when it can and tells you when it cannot.
Done on your device
Remuxing and converting both run inside your browser using ffmpeg compiled to WebAssembly. Nothing is uploaded, so there is no size cap on the file you start with — relevant here, because MKV files are frequently very large. Open your Network tab while it runs and you will see nothing leave.